This invention relates to systems and methods for controlling the temperature of individual units and subsystems in a system, and more particularly to a compact and versatile system and method for individually controlling the temperature of different tools in a semiconductor processing system.
Many production processes maintain control of the temperature of individual units or elements within an overall system by refrigerating or heating the individual units during operation of the system. A particularly noteworthy and critical example of the type of demanding environment in which precise temperature control is needed is found in semiconductor fabricating processes. The manufacture of integrated circuits by forming multiple replicated patterns on semiconductor wafers involves numerous successive steps. Following image replication, intense energy concentrations are used to etch, deposit and otherwise treat successive layers on the wafer, but at the same time precise placement, alignment and dimensional stability of the wafer must be maintained during practically all such process steps. Furthermore the final product cannot accept even minuscule defects even though temperature differentials can tend to distort wafers, affect alignments and deteriorate pre-existing layers. Given these and other considerations, semiconductor fabrication facilities require vast capital expenditures to provide tools and support equipment meeting the conflicting demands of quality control and high volume output at the levels of resolution now demanded by the state of the art.
An example of one type of semiconductor fabrication equipment now in wide use is the cluster tool, in which different closely juxtaposed tools are used, singly or in combination, to transport, position, and complete different ones of a succession of processing steps quickly and efficiently. The tools within the cluster can vary widely in purpose and function. Some tools in the cluster may have to be refrigerated at times to levels as low as xe2x88x9240xc2x0 C., while at the same time others may have to be heated to levels as high as 80xc2x0 C. to 100xc2x0 C. The levels will vary during a process, but at any given time, the then chosen temperature level must be maintained closely at each operative tool. In addition, abrupt temperature changes are sometimes needed. For instance, extremely rapid cooldown of a tool at a transition point in a procedure may mean that the overall process can be significantly shortened or substantially more efficient. If transition times can be markedly shortened for tools when they are within an evacuated process chamber, the same chamber can be used again for a different step, without the need for returning the chamber to ambient pressure and reestablishing the vacuum, or utilizing a second chamber for the subsequent processing step.
The different tools in a cluster have heretofore largely been refrigerated or heated by individual units, each using a separate prime refrigeration source with a compressor/condenser system, or a separate heater. This not only affects reliability by increasing the number of critical and active operating units, but also requires that a substantial amount of floor space be dedicated to the cluster tool. However, every square foot of area required in a semiconductor fabrication facility is extremely costly. A large xe2x80x9cfootprintxe2x80x9d size thus imposes a substantial economic penalty. Modern temperature control units for a cluster tool having, for example, six tools, require in the range of 18 square feet or more of floor area. Moreover, the service lifetime of these systems is limited because of the need to use multiple small refrigeration units, since they have shorter lifetimes than larger units and offer more chances of failure.
On-line maintenance of cluster tools requires that they be flushed of heat transfer fluid and disconnected from the temperature control unit. Current approaches typically use quick disconnects, which allow fluid to spill, and which tend to leak after a number of operations and impose a significant pressure drop in the system. An efficient subsystem for flushing and filling fluid used in temperature control is therefore highly desirable.
Precise control of the temperature of refrigerant-cooled fluid over a long service life is a desirable goal seldom achieved in practice. Solenoid valves, bimetallic proportional valves and other controls often used have inherent wear and hysteresis problems which affect both their accuracy and long-term life. Thermal expansion valves are capable of better resolution and proportional control, but present new problems when used in a refrigeration system for cluster tools, since it is the tool temperature which must be regulated, not superheat as in prior systems. In addition, prior thermal expansion valve systems are not usually able to effect extremely rapid cooldown because of thermal inertia problems.
A system in accordance with the invention concurrently operates a number of thermal control channels for thermal transfer fluid, each channel having both cooling and heating capability for adjusting the temperature of an associated process device. This system advantageously serves as a compact multi-channel temperature control unit for the different tools in a cluster tool.
In accordance with the invention, individual tools in a cluster tool used in semiconductor wafer processing are differently temperature controlled by control loops circulating selectively heated or cooled thermal transfer fluid. The control loops respond to sensors which measure actual tool temperatures and include circuits for providing the signals for regulating thermal transfer fluid temperature. Those channels requiring significant chilling receive a subcooled, high-pressure refrigerant from a single refrigerator unit having a total refrigeration capacity sufficient to meet the total demands of all the tools. A high pressure refrigerant, after compression, condensation and subcooling, is fed out in separate lines to the different control loops, where pressurized heat transfer fluid is circulated through the tools. The control of refrigerant flow to each different evaporator in the loop determines the amount of refrigeration capacity used to cool the heat transfer fluid and hence the tool. Thereafter, the expanded refrigerant is returned to the single refrigeration unit.
Further, in accordance with the invention, all channels in a multi-channel system circulate thermal transfer fluid replenished from a single pressurized tank in separate control loops including the pressurizing pumps, heat exchangers and heaters. The low temperature channels incorporate compact evaporator/heat exchange units controlled by thermal expansion valve units which are arranged to respond to tool temperature and also to prevent astable operation. Where only moderate cooling need be supplied to a tool, the heat exchangers in the respective channels receive only utilities water or other cooling medium at some ambient temperature. The heaters in the control loops are energized to raise the temperature of the heat transfer fluid when the tools are to be operated at temperatures above ambient.
The channels are configured with flow control valves which allow disconnection of the lines to any tool for service. The lines for heat transfer fluid that couple to and from the tool are disposed with at least one low point below the tool level, and include purge valves coupled to the low point and the vicinity of the tool. The temperature control unit includes separate lines coupled to the pressurized tank from each control channel. A pressurized source of purge gas is usually readily available for use in flushing. The entire loop section that includes the lines to and from the tool and the tool itself is effectively purged by using the purge gas to force heat transfer fluid through the tool from the low point and into the pressurized tank. The return line can further be purged in an opposite direction back through the return coupling to the pressurized tank. The tool can then be serviced or replaced and reconnected. To refill the lines and the tool with heat transfer fluid, the pressurized fluid source, which may be augmented by purge gas pressure, is coupled into the lines and into the tool as the flow control valves are held open and a point is vented to atmosphere. This mode of operation and configuration greatly facilitate the flush and fill functions, both in terms of reducing time and eliminating the often substantial leakage and dispersion of heat transfer fluid around the area.
A feature of this arrangement is that only a single large refrigeration system in used to cool a fluid in the different number of channels. Such an approach greatly reduces the parts count, while increasing the reliability of the system because large refrigeration systems are inherently more reliable than smaller units. In addition, the single refrigeration unit incorporates elements and subunits which improve its efficiency and reliability, such as a subcooler for interchanging thermal energy with low pressure suction return gases, an injection capillary system, a hot gas bypass and a superheater expansion valve system, as well as gas filter and oil separator devices. There is more efficient use of space because the total volume required is less than what individual units would require for the same capacity.
In addition, this system substantially reduces the critical floor space area that is needed because all the subsystems may be compactly disposed within an open sided frame having a small area base. An array of pumps and closely spaced in-line motors are mounted within the upper part of the frame. The compressor for a high capacity, high efficiency refrigeration system is disposed within the frame alongside and lower than the pump/motor array. Other units in the refrigeration system are disposed below and alongside the array. An accumulator vessel and a pressurized tank for heat transfer fluid are placed at opposite sides of the frame. A row of heat exchangers and flow controls for the chilled loops are disposed side by side below the array. Inlet and outlet ports and conduits for interconnection to the lines running to the tools and utility water are all accessible at an open side of the frame. The pump/motor combinations for the chilled channels can be disposed in the interior of the frame, for better isolation from ambient temperature effects. With the inlets, outlets and valves all accessible from the one side, manual connections, including manual connections of fluid and gas lines, and manual controls are all conveniently available. Manifolds are used to distribute cooling water, refrigerant, and heat exchange fluid from common sources to the different channels. For the unchilled channels, small heat exchangers are disposed on the opposite side from the open frame, alongside the pump/motor array. The replenishment and purge line are small and woven throughout the system. All chilled units and lines are covered with insulation extending out to the exterior tools. The footprint for a unit having three total channels, one or two of which can be chilled to xe2x88x9240xc2x0 C., is less than 0.6 ft.2 per channel. A footprint of 12xe2x80x3xc3x9734xe2x80x3 has also been realized for a system with greater than 4000 watts cooling capacity at xe2x88x9240xc2x0 C., and which includes three moderate temperature and three low temperature channels.
The proportional valve systems are of a long lifetime thermal expansion valve type that has no wearing or frictional parts and provides precise proportional control. However, the tool temperature that is sensed is not directly controlled, since in this system, control is at the input to the evaporator/heat exchanger. If the sensed tool temperature demands a response beyond the capacity of the evaporator, then the evaporator efficiency may enter an astable phase in which evaporator output increases rather than declines, so that further valve opening decreases rather than increases chilling. The evaporator output may also contain liquid refrigerant, a condition to be avoided at the compressor. The problem is obviated in one example by employing a reference evaporator in parallel to the chilled channels, and controllably heating pressure bulbs thermally coupled to the reference evaporator but communicating with flow control valves in the different channels. In addition, parallel flows from the evaporator outputs may be combined and the return flow temperature to the refrigeration unit used to limit flow to assure that excessive refrigerant flow will not be demanded. In a second version of an improved thermal expansion valve system for controlling multiple channels, superheat temperatures are measured at each evaporator, including the reference evaporator, and the controller is used to diminish heater temperatures in inverse relation to the difference between the individual evaporator output levels and the reference evaporator output level.